Scintillation detectors have been employed in the oil and gas industry for well logging. These detectors have used thallium-activated sodium iodide crystals that are effective in detecting gamma rays. The crystals are enclosed in tubes or casings to form a crystal package. The crystal package has an optical window at one end of the casing which permits radiation-induced scintillation light to pass out of the crystal package for measurement by a light sensing device such as a photomultiplier tube coupled to the crystal package. The photomultiplier tube converts the light photons emitted from the crystal into electrical pulses that are shaped and digitized by associated electronics. Pulses that exceed a threshold level are registered as counts that may be transmitted "uphole" to analyzing equipment or stored locally.
The ability to detect gamma rays makes it possible to analyze rock strata surrounding the bore holes, as by measuring the gamma rays coming from naturally occurring radioisotopes in down-hole shales which bound hydrocarbon reservoirs. Today, a common practice is to make measurements while drilling (MWD). For MWD applications, the detector must be capable of withstanding high temperatures and also must have high shock resistance. At the same time, there is a need to maintain performance specifications.
As new MWD tools are developed, the need for smaller detectors that meet or exceed larger detector performance is paramount. Current geophysical detectors that use hygroscopic crystals, such as thallium-activated sodium iodide crystals, require that the crystal be hermetically sealed in a stainless steel container. In order to maintain that seal under operating conditions, typically a soda lime glass window is hermetically sealed to the stainless steel housing by means of a glass to metal seal. The window is required to transmit the scintillated light produced in the crystal to a light sensing device such as a photomultiplier tube. This window assembly, along with the multiple optical interfaces needed, degrades the light transmitted to the photomultiplier. It follows, if the window and the associated interface can be removed, a gain in optical performance can be realized. This translates into a smaller crystal that has increased system nuclear performance of a larger crystal having an interface/window assembly. Therefore, it is desirable to have the photomultiplier tube directly coupled to the crystal and hermetically sealed in the housing.
However, there are many problems that must be addressed in the construction of such a windowless detector. These problems include the hermeticicity of the electrical pass-throughs, the off-gassing of volatile components that may degrade the hygroscopic crystal, and the survivability of the device under extreme environmental conditions.
Accordingly, it will be understood from the above that it would be desirable to have a scintillation detector without an optical window which overcomes the above problems.